The unit hertz (Hz) is a measure of frequency, with 1 hertz indicating that whatever you're measuring occurs once per second; for example, if your computer screen refreshes itself 40 times per second, the refresh rate would be 40 hertz. Horsepower (HP) is a unit of power, the amount of work performed over a period of time. Since these two units don't measure the same thing, they can't be directly converted. However, if you have a certain amount of work occurring at a certain frequency, you can state that in terms of horsepower.
Check that you have a measure of work and a measure of frequency. Work is often measured in foot-pounds (ft-lbs, standard), Joules (J, metric) or Newton-meters (N-m, equivalent to a Joule); frequency is measured in hertz. If your work is measured in foot-pounds, skip to Step 3.
Convert your work in Joules (or the equivalent Newton-meter) to foot-pounds. Divide the amount of work in Joules by the conversion rate: 1 J = 0.737562149 ft-lbs.
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Convert your work and frequency to power. To do this, take your work (in foot-pounds) and the frequency at which that work occurs (in hertz) and simply multiply the two values together.
You now have a unit of power called foot-pounds per second (ft-lbs/s). To convert this unit to mechanical horsepower, divide it by the conversion rate: 1 HP = 550 ft-lbs/s
Mechanical horsepower is used in the United States and England, but in the rest of Europe, a slightly different unit of measurement called metric horsepower is used. To convert to metric, divide your power in mechanical horsepower by the conversion rate: 1 mechanical HP = 1.01387 metric HP
There are other types of horsepower for different situations, including boiler horsepower and electric horsepower (see References for more information).